Sulphur pollution near one of Hong Kong’s busiest shipping lanes fell markedly in the first week of this month as a result of new regulations mandating ocean-going vessels switch to cleaner fuel, according to the Clean Air Network.

Average 24-hour concentrations of toxic sulphur dioxide (SO2) in Kwai Chung were recorded at 12 micrograms per cubic metre of air between July 1 – the day the mandate went into effect – and Tuesday.

By comparison, the average 24-hour SO2 concentration in the same period last year was 34 micrograms per cubic metre and 23 micrograms per cubic metre the year before that.

The new rule requires all ocean-bound vessels from tugboats to container ships to switch to 0.5 per cent sulphur marine fuel when berthing in the city.

Kwai Chung, which together with Tsing Yi forms Kwai Tsing district, is located near the Kwai Chung container port – the world’s fourth biggest in terms of throughput.

Previous studies have found the area to be the worst hit district in the city from ship pollution. Ships are the biggest source of SO2 in the city, followed by power generation.

In terms of roadside pollution, average levels of nitrogen dioxide, suspended particulates and ozone all dropped in the first half of the year.

But average concentrations of microscopic particulate matters suspended in the air, or PM, rose in Tuen Mun, Tung Chung, Causeway Bay and Mong Kok in the same period. Particulates can penetrate into the lungs and cause respiratory and cardiovascular problems.

All five major pollutants measured at 11 out of 15 of the city’s air quality monitoring stations exceeded the World Health Organisation’s guidelines.

Other than in Kwai Chung, Kwun Tong and Tai Po, concentrations of ozone – an indicator of regional air quality –recorded lower than average recordings at all ambient monitoring stations in the first six months of the year.

But the number of times ozone levels exceeded WHO guidelines over eight-hour periods rose at all stations in the same period.